The present invention is related to a wire loading device in a paper machine, by whose means a mechanical load is applied to the wire of the paper machine, preferably across its entire width, and by means of which load a pressure pulse is applied to the fibre layer or web placed on support of a wire or between wires in order to promote the dewatering of the web, improve the formation of the web, and/or to control the transverse profiles of different properties of the web, such as the transverse profiles of dewatering, filler distribution, formation, and/or retention.
In the web forming sections of paper machines, a number of different forming members are employed. The principal function of these members is to provide a pressure pulsation in the fibre layer that is being formed, by means of which pulsation the dewatering of the web that is formed is promoted and, at the same time, its formation is improved. Also, in prior art, a number of different forming shoes are known, which are usually provided with a curved ribbed deck and over which the forming wires placed one above the other and the web placed between said wires are made to curve. In the area of these forming shoes, water is removed primarily through the web placed at the side of the outside curve, because of its tensioning pressure, and this dewatering is aided by the field of centrifugal force. The ribbed deck of the forming shoe produces pressure pulsation, which promotes the dewatering and improves the formation of the web.
In the prior art, various suction boxes and suction rolls, register rolls, forming ribs and foil ribs and doctors are known, by means of which a difference in pressure and pressure pulsation are produced in the fibre layer that is being formed, thus promoting the web formation and the dewatering.
For example, reference is made to U.S. Pat. Nos. 2,881,676 and 3,438,853 and to Patent Application WO 91/02842.
Attempts are made to construct the headboxes and the web forming components of a paper machine so that it should be possible to produce a paper that has a grammage, formation, and strength properties homogeneous across the entire width of the web, so that a minimum proportion of the paper at the edges of the web has to be cut off. From paper, in particular from fine paper, an ever higher homogeneity of the structure is required, which is required by printing and copying methods in which very rapid and intensive heating of the sheet takes place.
In order that homogeneous paper can be manufactured, an important requirement is that the main axes of the directional distribution, i.e. orientation, of the fiber mesh in the paper coincide with the directions of the main axes of the paper and that the orientation is symmetric with respect to these axes. For example, in the case of copying papers, it is important that the orientations at their top and bottom faces are substantially the same.
In view of achieving said objectives, as is known in prior art, various solutions for the control of the pulp suspension flows in a headbox have been employed, in whose respect reference is made by way of example to the assignee's FI Patent No. 75,377. However, also after the headbox, it is necessary to control the transverse homogeneity of the paper produced by various means, e.g. the transverse profile of the web in respect of dewatering, distribution of fillers, and retention.
One of the drawbacks with the use of the prior-art forming members is wire damage, which results from particles of impurities passing between the glide faces of the forming members and the wires, which particles may cause flattening and/or shifting of wire fibers, with resulting wire damage. This drawback occurs with particular emphasis when there are two forming members, such as forming ribs, placed one against the other in contact with the two wires, "hard against hard", the wires and the fiber web between them having to run between said forming ribs.
As a rule, the constructions of the prior-art web forming members are heavy, and a transverse bending occurs in said members, which drawback is increased to a great extent when the widths of paper machines are increased. The transverse profiling of the intensity of the pressure pulsation produced by said prior-art members is usually not possible without costly special arrangements.
The constantly increasing running speeds of paper machines have also brought about ever increasing requirements on the different web forming members.